Tech Talk # 43 – The Trouble with Flat Tappets

Faithful readers of this column know that my mantra on engine building is to spend your money wisely. Sometimes that means writing a bigger check initially in order to save money in the long run. A case in point is the decision whether to use flat tappets or roller lifters. In my view, a flat tappet is a ticking time bomb in a racing engine. Sooner or later, that bomb is going to explode with expensive results.

I’ll climb up on my soapbox and tell the drag racing world that a flat tappet camshaft is a bad investment for any serious competition engine. A typical flat tappet valvetrain may have a lower up-front cost than a roller camshaft, but it’s going to cost much more over the engine’s lifetime. The odds are that a flat tappet camshaft is either going to fail outright or cause a dropped valve. In either case, you’re looking at a thoroughly trashed engine and a major repair bill.

I know that some racers have run flat tappet cams successfully for years, and that NASCAR Nextel Cup teams are required by the rules to use flat tappets. The former are very fortunate, while the latter spend mountains of money on special cam cores made from exotic alloys, hardened and inlaid lobe faces, oversize zero-radius lifters, camshaft oilers and major block modifications to eke out 500 miles of racing from a flat tappet cam before it self-destructs. Flat tappets don’t save money for NASCAR teams – in fact, the total cost far exceeds the price of a roller cam and lifters. Moreover, the highly specialized components and machining procedures used in NASCAR Nextel Cup engines have virtually nothing in common with the flat tappet cams listed in retail catalogs.

Flat tappets survive in street engines because the valve spring pressure is relatively low. Consider that the seat pressure for a stock small-block Chevy V-8 with a flat tappet cam is 85 pounds. The generally accepted maximum spring pressure for a flat tappet cam used in competition is 130 pounds. That’s still much less than the 300+ pounds of seat pressure typically used with roller lifters in racing engines. Moreover, a low-tension flat tappet valve spring and a heavy steel valve is the worst possible combination for high-rpm reliability. The valve’s inertia can easily overcome the weak valve spring tension, producing violent, uncontrolled motion that can lead to a dropped valve and catastrophic failure.

Installing a flat tappet cam correctly demands considerable effort by the engine builder. Doing the job right requires making sure that every cam lobe is precisely positioned in relation to its respective lifter. The lobes on flat tappet camshafts are intentionally ground with a slight taper that causes the lifters to spin; if a lifter doesn’t rotate because of improper offset, the resulting friction will quickly destroy the lobe. The metal worn off the cam lobe is then embedded in the piston skirts and cylinder walls, causing scuffing and serious cylinder wall damage.

In contrast, a roller lifter can tolerate slight misalignment in the position and angle of the lifter bores. A roller bearing also requires much less lubrication than the sliding base of a flat tappet, so the amount of oil circulating in the engine can be reduced to minimize windage losses.

A flat tappet severely limits the camshaft profile and thereby restricts the engine’s performance potential. The diameter of a flat tappet dictates how quickly the valves can be accelerated. Years ago engine builders replaced standard .842-inch diameter GM lifters with larger .940-inch tappets and special mushroom lifters because the increase in lifter diameter allowed more area under the lift curve. But even the fattest flat tappets can’t produce the same valve acceleration as a standard diameter roller lifter. Consequently flat tappet cam profiles have long seat timing that bleeds off cylinder pressure without delivering the breathing benefits of increased duration.

In my experience, the people who tend to favor flat tappets are either newcomers to racing who are trying to save money or veterans who had a bad experience with roller lifters in the distant past. I think that both groups are making a grave mistake by not using a roller cam unless they compete in a class that specifically requires flat tappets. Yes, a roller cam and kit does cost more than a flat tappet cam and a set of lifters, but the cost of fixing an engine after flattening a cam or dropping a valve is much more expensive. It’s true that a roller lifter may fail occasionally – usually as the result of a broken valve spring or incorrect valve lash adjustment – but the likelihood of trouble is much less than with flat tappets.

I’m a frugal person, as anyone who has seen our Pro Stock racing operation can attest. My philosophy on engine building is that it’s much cheaper to do a job right the first time than to do it wrong and then fix it. It always costs more to repair something, and generally the effort and expense are a lot more than you ever imagined. For my money, flat tappets belong in the museum, not on the race track.